Less than six percent of North Americans suffer from vitamin D deficiency, but nearly 20 percent take vitamin D pills. It is true that it is difficult to get adequate levels of vitamin D from sunshine during the winter months, but vitamin D is not a miracle vitamin that treats and prevents all sorts of diseases. A recent study of 5,108 men and women showed that vitamin D pills did not help to prevent heart attacks (JAMA Cardiol, published online April 5, 2017), and another study showed that large doses of vitamin D did not help to prevent cancer (JAMA, Mar 28, 2017;317(12):1234-1243).

No Proven Benefit from High Doses of Vitamin D
The main function of vitamin D is to help keep bones strong and control calcium metabolism. Vitamin D increases calcium absorption in the intestines and strengthens bones by increasing bone remodeling and increasing the effects of parathyroid hormones. Almost all of the other reported functions of vitamin D are controversial.

There is no proven benefit from taking high doses of vitamin D (>1000 IU/day) or having very high blood levels (>20ng/ml). Raising blood levels of hydroxy vitamin D from 20 to 30 ng/ml with high doses of vitamin D pills increases calcium absorption by only one percent and does not increase bone mineral density or physical function, compared with placebo (Curr Opin Endocrinol Diabetes Obes, Dec 2016;23(6):440-444).

Confusion about Vitamin D Deficiency
It is difficult for doctors to identify patients with vitamin D deficiency because most commercial laboratories do not offer a test for the active form of vitamin D, 1,25 dihydroxy vitamin D. Instead, laboratories measure an inactive form called hydroxy vitamin D. The massive doses of vitamin D recommended by some doctors do not raise the active form of vitamin D because your own body tries to protect you from poisoning by breaking down the active form. You cannot develop vitamin D poisoning from too much sunlight because sunlight breaks down vitamin D just as it helps your body to make it, so as you keep exposing your skin to sunlight, blood levels reach a certain level and do not rise higher. Massive doses of vitamin D pills such as 150,000 IU every three months fail to raise blood levels of the active 1,25 hydroxy vitamin D ((J Adolesc Health, Jul, 2015;57(1):19-23).

Many studies show that people with dark skin have lower levels of the inactive form of vitamin D. It's a different story when doctors measure blood levels of the active form of vitamin D. People with dark skin are able to use a much higher proportion of the active form of vitamin D (N Engl J Med, Nov 21, 2013;369(21):1991-2000).

Current Guidelines
The Institute of Medicine recommends that adults need only 600–800 IU of vitamin D per day and that blood levels of hydroxy vitamin D do not need to be higher than 20 ng per milliliter (Curr Opin Endocrinol Diabetes Obes, Dec 2016;23(6):440-444). Higher blood levels of vitamin D (greater than 20 ng/ml) do not make bones stronger than lower blood levels as they do not reduce levels of parathyroid hormone or bone resorption (Curr Rheumatol Rep, June 2011;13(3):257-64).

Large doses (4000 IU/day) of vitamin D did not slow declining physical function in sedentary men over 70 (Journal of the American Geriatrics Society, 11/22/2016). Furthermore, for most people, high dose vitamin D pills (greater than 2000 IU/day) are not going to improve health and may even harm you (N Engl J Med, Nov 10, 2016;375:1817-1820).

My Recommendations
You probably do not need to take vitamin D pills if your blood level of hydroxy vitamin D is above 20 ng/ml unless you have a condition that your doctor feels puts you at increased risk for the signs and symptoms of a deficiency. You can take up to 1000 IU/day of vitamin D pills if your blood levels are below 20 ng/ml. Most researchers and clinicians now feel that, with few exceptions, high doses of vitamin D are not beneficial and are potentially harmful. You should not take doses of vitamin D greater than 1000 IU/day unless your doctor diagnoses a specific reason to do so. People who could possibly benefit from treating low blood levels of hydroxy vitamin D (<20 ng/ml) include those who:
• are inactive and do not go outdoors
• suffer from generalized muscle and joint pain
• are athletes with recurrent injuries and decreased performance
• have weak bones (osteoporosis)
• are diabetic, particularly if LDL cholesterol is over 100
• have an auto-immune disease
• are critically ill or debilitated
• suffer from muscle pain from taking statin drugs.

About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities.
Read More